The present invention relates to an oil-resistant rubber composition or, more particularly, to a rubber composition comprising, as the principal polymeric constituents, an acrylic copolymer and a copolymer of ethylene and propylene or ethylene, propylene and a dienic monomer and capable of giving a covulcanizate highly resistant against oils.
Known acrylic rubbers include those of a linear polymer obtained by the copolymerization of ethyl acrylate as the principal comonomer with butyl acrylate and methoxyethyl acrylate. These conventional acrylic rubbers are excellent in their oil resistance and heat resistance while they are used practically only in some limited applications because they are not always excellent in respect of the susceptibility to hydrolysis, resistance against chemicals and solvents and electric properties as well as mechanical properties as a rubber such as the rubbery elasticity, recoverability from compression and mechanical strengths.
On the other hand, rubbers based on a copolymer of ethylene and propylene, referred to as EPM hereinbelow, or a copolymer of ethylene, propylene and a dienic monomer, referred to as EPDM hereinbelow, are excellent in their heat resistance, weatherability and electric properties so that they are widely used as gaskets in buildings, roofing materials, automobile parts, insulation of electric wires and so on while they have a disadvantage of the poor resistance against oils and chemicals so that improvements in this regard are eagerly desired for these rubbers. The inventors have previously proposed a covulcanizate of an EPM or EPDM rubber with a silicone rubber (see, for example, Japanese patent publication No. 57-17011) although no quite satisfactory results could be obtained in respect of the oil resistance of the rubber vulcanizate.
It would be an idea that a covulcanizate of an acrylic rubber and an EPM or EPDM rubber may have satisfactory properties as a combination of these two types of rubbers. This idea of covulcanization is, however, generally understood not to be practical due to the difference in the mechanisms involved in the vulcanization of these rubbers of different types. Namely, the functional groups in the acrylic rubbers are --CH.sub.2 --CH(OC.sub.2 H.sub.4 Cl)--, --CH.sub.2 --CH(OCOCH.sub.2 Cl)--, --CH.sub.2 --CH(CH.sub.2 --O--Gl)-- and the like, Gl being a glycidyl group, to pertain to the crosslinking reaction and acrylic rubbers are usually vulcanized with a polyamine as the vulcanizing agent while EPM or EPDM rubbers cannot be vulcanized with a polyamine but should be vulcanized with sulfur or an organic peroxide. Therefore, no satisfactory covulcanizate can be obtained of a mere blend of these two types of rubbers.